Mossbauer and EPR studies of hydrogenases isolated from Clostridium pasteurianum W5 and from sulfate reducing bacteria are proposed. Hydrogenase is capable of activating molecular hydrogen and many micro-organisms use it to metabolize H2. Understanding the hydrogenase mechanism could be a key step toward catalysis of hydrogen production from water and solar energy, a potentially important alternate energy source. Despite the importance of its biological functions, hidrogenase is poorly understood. The proposed studies should reveal physical properties and structural information pertinent to the prosthetic groups of hydrogenase; information essential to the understanding of its catalytic mechanism. Our preliminary studies have acquired evidence against the present understanding of a HiPIP center in C. pasteurianum hydrogenase, and have established unambiguously the presence of a redox-active nickel center in hydrogenases form Desulfovibrio gigas and Desulfovibrio desulfuricans. Since nickel has only been recently recognized as an important trace element in biological systems. Our studies will be undoubtedly an important step toward the understanding of biological functions of nickel in proteins.